Shareholder Report, Holdings (Details)	Jun. 30, 2025
C000005520 \| U.S. Treasury Notes
Holdings [Line Items]
Percent of Net Asset Value	17.10%
C000005520 \| Federal National Mortgage Assn.
Holdings [Line Items]
Percent of Net Asset Value	4.00%
C000005520 \| Federal Home Loan Mortgage
Holdings [Line Items]
Percent of Net Asset Value	2.10%
C000005520 \| Government National Mortgage Assn.
Holdings [Line Items]
Percent of Net Asset Value	1.00%
C000005520 \| CVS Health
Holdings [Line Items]
Percent of Net Asset Value	0.80%
C000005520 \| HCA
Holdings [Line Items]
Percent of Net Asset Value	0.70%
C000005520 \| Wells Fargo
Holdings [Line Items]
Percent of Net Asset Value	0.70%
C000005520 \| Goldman Sachs Group
Holdings [Line Items]
Percent of Net Asset Value	0.70%
C000005520 \| Crown Castle
Holdings [Line Items]
Percent of Net Asset Value	0.70%
C000005520 \| SBA Tower Trust
Holdings [Line Items]
Percent of Net Asset Value	0.60%
C000005520 \| Corporate Bonds
Holdings [Line Items]
Percent of Net Asset Value	43.50%
C000005520 \| Asset-Backed Securities
Holdings [Line Items]
Percent of Net Asset Value	20.60%
C000005520 \| U.S. Government Agency Obligations (Excluding Mortgage-Backed)
Holdings [Line Items]
Percent of Net Asset Value	17.60%
C000005520 \| Non-U.S. Government Mortgage-Backed Securities
Holdings [Line Items]
Percent of Net Asset Value	7.30%
C000005520 \| U.S. Government & Agency Mortgage-Backed Securities
Holdings [Line Items]
Percent of Net Asset Value	7.10%
C000005520 \| Foreign Government Obligations & Municipalities
Holdings [Line Items]
Percent of Net Asset Value	1.90%
C000005520 \| Commercial Paper
Holdings [Line Items]
Percent of Net Asset Value	1.10%
C000005520 \| Securities Lending Collateral
Holdings [Line Items]
Percent of Net Asset Value	0.40%
C000005520 \| Other Holdings Grouping
Holdings [Line Items]
Percent of Net Asset Value	0.50%
C000005521 \| U.S. Treasury Notes
Holdings [Line Items]
Percent of Net Asset Value	17.10%
C000005521 \| Federal National Mortgage Assn.
Holdings [Line Items]
Percent of Net Asset Value	4.00%
C000005521 \| Federal Home Loan Mortgage
Holdings [Line Items]
Percent of Net Asset Value	2.10%
C000005521 \| Government National Mortgage Assn.
Holdings [Line Items]
Percent of Net Asset Value	1.00%
C000005521 \| CVS Health
Holdings [Line Items]
Percent of Net Asset Value	0.80%
C000005521 \| HCA
Holdings [Line Items]
Percent of Net Asset Value	0.70%
C000005521 \| Wells Fargo
Holdings [Line Items]
Percent of Net Asset Value	0.70%
C000005521 \| Goldman Sachs Group
Holdings [Line Items]
Percent of Net Asset Value	0.70%
C000005521 \| Crown Castle
Holdings [Line Items]
Percent of Net Asset Value	0.70%
C000005521 \| SBA Tower Trust
Holdings [Line Items]
Percent of Net Asset Value	0.60%
C000005521 \| Corporate Bonds
Holdings [Line Items]
Percent of Net Asset Value	43.50%
C000005521 \| Asset-Backed Securities
Holdings [Line Items]
Percent of Net Asset Value	20.60%
C000005521 \| U.S. Government Agency Obligations (Excluding Mortgage-Backed)
Holdings [Line Items]
Percent of Net Asset Value	17.60%
C000005521 \| Non-U.S. Government Mortgage-Backed Securities
Holdings [Line Items]
Percent of Net Asset Value	7.30%
C000005521 \| U.S. Government & Agency Mortgage-Backed Securities
Holdings [Line Items]
Percent of Net Asset Value	7.10%
C000005521 \| Foreign Government Obligations & Municipalities
Holdings [Line Items]
Percent of Net Asset Value	1.90%
C000005521 \| Commercial Paper
Holdings [Line Items]
Percent of Net Asset Value	1.10%
C000005521 \| Securities Lending Collateral
Holdings [Line Items]
Percent of Net Asset Value	0.40%
C000005521 \| Other Holdings Grouping
Holdings [Line Items]
Percent of Net Asset Value	0.50%